The present invention pertains to a heating device for webs to be unwound from rolls, comprising a holder for rotatably supporting a roll with a web wound up on it and a heater for acting on an unwound area of said web.
The present invention also pertains to a process for controlling the heating device.
During the shaping of plastics according to the so-called deep-drawing process, flat plastic webs or films are transformed into simple three-dimensional molded bodies. The deformation process takes place in the heated state of the web.
In the known deep-drawing processes, the plastic webs are in rolls. The plastic web is unwound from the roll during the processing. The roll proper is not driven, but is set into rotary movement by the plastics processing station in the course of the work rhythm by timed pulling of the web. The roll is consequently set into jerky rotary movement. since the mass of the web put into rotary movement decreases during the unwinding, pulling of the web even with constant force does not lead to equal angles of rotation of the roll. Therefore, the heater disposed between the roll and the processing center, which serves to heat the unwound web, may act over different lengths of time on different segments of the web. This in turn leads to the amount of heat to be applied to the web not being able to be exactly determined in advance. As a consequence of this, the web is not heated uniformly, which leads to impairment of quality in the deep-drawing product.